The present invention generally relates to magnetic heads and more particularly to a high-sensitivity magnetic head that uses a so-called ferromagnetic tunnel-junction.
Magnetic heads are used extensively in magnetic storage devices ranging from audio-visual apparatuses such as a video recorder or tape recorder to information processing apparatuses such as a computer. In information processing apparatuses in particular, there is a need of processing a very large amount of information in relation to processing of image data or audio data, and associated therewith, there is a demand for a large-capacity high-speed magnetic storage device that is capable of recording information with a very large recording density.
The recording density of information that can be read out by a magnetic head, or resolution limit, is determined primarily by a gap width of the magnetic head and a distance of the magnetic head from the recording medium. In an induction type magnetic head in which a magnetic core is wound around with a coil, a recording density of as much as 65 Mbits/inch.sup.2 is achieved when the gap-width is set to 1 .mu.m. On the other hand, there is a prediction that a magnetic head capable of performing reading and writing with a recording density exceeding 20 Gbits/inch.sup.2 is required in future. In order to achieve this, it is essential to provide a super high-sensitivity magnetic sensor that is capable of detecting a very feeble magnetic signal, while such a super high-sensitivity, super high-resolution and super high-speed magnetic sensor cannot be realized by an induction-type magnetic head that operates on the principle of electromagnetic induction.
As a high-sensitivity magnetic head capable of detecting such a very feeble magnetic signal caused by a very minute magnetic recording dot, there is a proposal of using a so-called MR (magneto-resistance) magnetic sensor that uses an anisotropic magneto-resistance or a GMR (giant magneto-resistance) magnetic sensor that uses a giant magneto-resistance, for the magnetic head.
FIG. 1 shows the construction of a typical conventional super high-resolution magnetic read/write head 10 in a cross-sectional view.
Referring to FIG. 1, the magnetic head 10 is constructed on a ceramic substrate 11 typically formed of Al.sub.2 O.sub.3.TiC, and the like, and includes a lower magnetic shield 12 formed on the substrate 11 and an upper magnetic shield 14 formed on the lower magnetic shield 12, with a non-magnetic insulation film 13 interposed between the lower magnetic shield 12 and the upper magnetic shield 14. The upper and lower magnetic shields 12 and 14 form a read gap 15 at a front edge of the magnetic head 10 and a magnetic sensor 16 is disposed in the read gap 15 thus formed.
Further, a magnetic pole 18 is formed on the upper magnetic shield 14, with a non-magnetic insulation film 17 interposed therebetween, and the magnetic pole 18 and the upper magnetic shield 14 form together a write gap 19 at a front edge part of the magnetic head 10. Further, a write coil pattern 17 is formed in the insulation film 12.
In the magnetic head 10 of FIG. 1, use of various GMR magnetic sensors such as a spin-valve magnetic sensor is proposed for the magnetic sensor 16. A spin-valve magnetic sensor is a magnetic sensor that includes a pinned layer of a ferromagnetic material such as NiFe or Co formed adjacent to an anti-ferromagnetic layer formed of FeMn, IrMn, RhMn, PtMn, PdPtMnN, and the like, and a free layer of a ferromagnetic material such as NiFe or Co is formed in exchange coupling with the pinned layer, with a non-magnetic layer such as a Cu layer interposed therebetween. It should be noted that the pinned layer has a magnetization fixed by the anti-ferromagnetic layer, while the free layer changes a direction of magnetization in response to an external magnetic field. The spin-valve magnetic sensor changes a resistance thereof in response to the angle formed by the magnetization of the free layer and the magnetization of the pinned layer.
However, such conventional GMR magnetic sensors have a common problem, associated with the structural feature thereof that a non-magnetic layer is formed adjacent to a ferromagnetic layer, that the magnetic head is vulnerable to a heat treatment process. It should be noted that a heat treatment process, typically conducted at a temperature between 250.degree. C. and 300.degree. C., is inevitable in the fabrication process of the magnetic head 10 of FIG. 1.
FIGS. 2A-2E show a typical fabrication process of the magnetic head 10 of FIG. 1.
Referring to FIG. 2A, a thin insulation film corresponding to the write gap 19 is formed on the upper magnetic shield 14 after a magnetic structure including the magnetic sensor 16 and the upper magnetic shield 14 is formed, and a resist pattern 17A is formed further on the foregoing insulation film.
Next, in the step of FIG. 2B, the structure of FIG. 2A is annealed at a temperature of 250-300.degree. C. such that the vertical front edge of the resist pattern 17A undergoes a reflowing. As a result of the reflowing, a curved slope surface is formed in the resist pattern 17A at the foregoing front edge.
Next, in the step of FIG. 2C, the coil pattern 17C is formed on the resist pattern 17A thus annealed, and another resist pattern 17B is formed on the resist pattern 17A so as to bury the coil pattern 17C. Further, an annealing process is applied to the structure of FIG. 2C in the step of FIG. 2D at a temperature of 250-300.degree. C. similarly to the step of FIG. 2B, and a vertical front edge of the resist pattern 17B undergoes a reflowing similarly to the step of FIG. 2B. As a result of the step of FIG. 2D, the resist pattern 17B also has a curved slope surface similarly to the resist pattern 17A.
Next, in the step of FIG. 2E, the magnetic pole 18 is formed on the structure of FIG. 2D.
In the foregoing processes of FIGS. 2A-2E in which resist patterns 17A and 17B experience reflowing after the magnetic sensor 16 is formed, it should be noted that the magnetic sensor 16 experiences the thermal annealing process at the temperature of 250-300.degree. C. twice, in the step of FIG. 2B and in the step of FIG. 2D, while it is known that the GMR magnetic sensor loses most of the large magneto-resistance change, characteristic to a GMR sensor, as a result of such a thermal annealing process. In the case of a spin-valve sensor that uses PtMn or PdPtMnN, and the like for the anti-ferromagnetic layer in particular, it should be noted that a further thermal annealing process is necessary at a temperature of 250.degree. C. or higher in order to crystallize the anti-ferromagnetic layer.
On the other hand, there is a proposal to use a ferromagnetic tunnel-junction magnetic sensor, in which a tunnel insulation film is interposed between a pair of ferromagnetic layers, for the magnetic sensor 16 of the magnetic head 10. A ferromagnetic tunnel junction magnetic sensor is expected to show a very large magneto-resistance change against a feeble magnetic field, even larger than that of a spin-valve GMR sensor, and is thought a promising magnetic sensor for such super high-resolution magnetic head 10.
FIGS. 3A and 3B show the principle of a ferromagnetic tunnel-junction sensor as used for the magnetic sensor 16.
Referring to FIGS. 3A and 3B, the magnetic sensor 16 includes a lower ferromagnetic layer 16A of NiFe or Co and an upper ferromagnetic layer 16B also of NiFe or Co, with a tunnel-insulation film 16C of AlO.sub.x interposed between the layers 16A and 16B with a thickness of several nanometers, and electrons having an upward spin direction and electrons having a downward spin direction are caused to flow through the tunnel-insulation film 16C in the form of a tunnel current, generally perpendicularly to the principal surface thereof.
In the state of FIG. 3A, in which there is no substantial external magnetic field, it can be seen that the direction of magnetization in the ferromagnetic layer 16A and the direction of magnetization in the ferromagnetic layer 16B are in an anti-parallel relationship as a result of exchange interaction established between the ferromagnetic layers 16A and 16B. In contrast, the magnetization in the layer 16A and the magnetization in the layer 16B are parallel in the state of FIG. 3B in which there is an external magnetic field H.
In the ferromagnetic tunnel-junction magnetic sensor of such a construction, it should be noted that the tunnel probability of the electrons changes depending on the magnetization of the upper and lower magnetic layers 16A and 16B, and a tunnel resistance R of the magnetic sensor is changed by the external magnetic field H according to a relationship EQU R=R.sub.s +(1/2).multidot..sub..DELTA. R(1-cos .theta.) (1)
wherein Rs represents the tunnel-resistance for a reference state in which the direction of magnetization is parallel in the magnetic layers 16A and 16B, .theta. represents the angle formed by the magnetization in the magnetic layer 16A and the magnetization in the magnetic layer 16B, and .sub..DELTA. R represents the difference in the tunnel-resistance R between the state in which magnetizations are parallel in the magnetic layers 16A and 16B and in which magnetizations are anti-parallel in the magnetic layers 16A and 16B. It should be noted that .sub..DELTA. R always has a positive value. Thereby, a ratio of tunnel-resistance change or MR ratio is defined as .sub..DELTA. R/R.
As can be understood from Eq.(1), the tunnel-resistance R becomes minimum when the magnetization of the magnetic layer 16A and the magnetization of the magnetic layer 16B are in a parallel relationship as in the case of FIG. 3A. Further, the tunnel-resistance R becomes maximum when the magnetization of the magnetic layer 16A and the magnetization of the magnetic layer 16B are in an anti-parallel relationship as in the case of FIG. 3B.
It should be noted that such a change of magneto-resistance is caused due to the fact that an electron current generally includes both upward electrons having an upward spin direction and downward electrons having a downward spin direction. In a non-magnetic body, the number of the upward electrons and the number of the downward electrons are generally equal and the non-magnetic body does not show magnetism. In the case of a ferromagnetic body, on the other hand, the number of the upward electrons and the number of the downward electrons are different and this is the reason the ferromagnetic body shows magnetism.
When a single electron tunnels through the tunnel insulation film 16C from the ferromagnetic layer 16A to 16B or vice versa, the spin state of the electron is preserved before and after the tunneling. This also means that, in order that the electron tunnels from one ferromagnetic layer to the other ferromagnetic layer successfully, it is necessary that the other ferromagnetic layer includes a vacant state corresponding to the spin state of the foregoing electron. When there is no such a vacant sate, the tunneling of the electron does not occur.
It should be noted that the MR ratio .sub..DELTA. R/R of the tunnel resistance is represented as a product of the spin polarization of the electrons in the source ferromagnetic layer 16B and the spin polarization of electrons in the target ferromagnetic layer 16A, according to the relationship EQU .sub..DELTA. R/R=2P.sub.1 .multidot.P.sub.2 /(1-P.sub.1 .multidot.P.sub.2)
wherein P.sub.1 and P.sub.2 are represented according to the relationship EQU P.sub.1,P.sub.2 =2(N.sub.UP -N.sub.DOWN)/(N.sub.UP +N.sub.DOWN)
where N.sub.UP represents the number of up-spin electrons or the number of states of the up-spin electrons, while N.sub.DOWN represents the number of down-spin electrons or the number of states of the down-spin electrons in the ferromagnetic layer 16A or ferromagnetic layer 16B.
While the value of the spin polarization P.sub.1 or P.sub.2 generally depends on the ferromagnetic material, there are cases in which an MR ratio of as much as 50% is theoretically predicted when a proper material system is chosen. This value of the MR ratio is much larger than the MR ratio achieved by a conventional GMR sensor. Thus, a ferromagnetic tunnel-junction sensor is thought to be a promising magnetic sensor for use in a super high-resolution magnetic head. See for example the Japanese Laid-Open Patent Publication 4-103014.
Contrary to the foregoing prediction, the MR ratio achieved in conventional ferromagnetic tunnel-junction sensors is far smaller than the foregoing theoretical prediction. In fact, there are only a few examples in which a successful achievement of the MR ratio of 20% is reported (in room temperature environment). Even in such successful examples, there is a tendency that the MR ratio decreases with time or the device shows a too small withstand voltage for a reliable detection of the magneto-resistance change. The reason of this unsatisfactory result is thought to be caused by defects that are formed at the interface between the ferromagnetic layer 16A or 16B and the extremely thin tunnel insulation film 16C by a contamination of particles.
Conventionally, it is practiced to form the extremely thin tunnel insulation film 16C by depositing an Al layer on the lower magnetic layer 16A by a sputtering process with a thickness of about 5 nm (50 .ANG.) and further by applying an oxidation process to convert the deposited Al layer to a layer having a composition of AlO.sub.x (T. Miyazaki and N. Tezuka, J. Magn. Mater. 139, 1995, L231). However, the ferromagnetic tunnel-junction of such a conventional construction has a drawback, associated with the fact that the non-magnetic Al layer carrying thereon the insulation film 16C has a substantial thickness, that the obtained MR ratio is small.
Further, there has been a problem, in the conventional magnetic head 10 including a ferromagnetic tunnel-junction sensor for the magnetic sensor 16 and fabricated according to the process of FIGS. 2A-2E, in that the foregoing Al layer may cause a reaction with the underlying ferromagnetic layer 16A in the thermal annealing process of FIG. 2B or 2D. When such a reaction is caused, there occurs a formation of non-magnetic solid solution in the ferromagnetic layer 16A, while the existence of such a non-magnetic region in the ferromagnetic layer 16A deteriorates the MR ratio substantially. Thus, the foregoing conventional ferromagnetic tunnel-junction sensor has suffered from the problem of vulnerability to thermal annealing processes similarly to conventional GMR magnetic sensors.
FIG. 4 shows the magnetization M.sub.s of a structure in which an Al layer is deposited on a Co layer having a thickness of 3 nm, with a thickness of 10 nm, for the case in which the thermal annealing temperature is changed variously.
Referring to FIG. 4, it can be seen that the magnetization Ms decreases with increasing thermal annealing temperature, indicating the occurrence of dissolution of non-magnetic Al atoms into the Co layer from the Al layer.
The result of FIG. 4 indicates that, when a substantial amount of non-magnetic atoms have dissolved from the tunnel insulation film 16C into the underlying ferromagnetic layer 16A in the ferromagnetic tunnel-junction sensor 16 of FIGS. 3A and 3B as a result of the thermal annealing process, the electrons in the upper ferromagnetic layer 16A can cause a tunneling to the lower ferromagnetic layer 16B irrespective of magnetization of the ferromagnetic layer 16B. Thereby, the MR ratio decreases substantially.
In order to improve the heat resistance of the ferromagnetic tunnel-junction sensor, Japanese Laid-Open Patent Publication 4-103013 teaches use of a IIIb-Vb compound for the tunnel insulation film. However, the ferromagnetic tunnel-junction sensor of the foregoing prior art can provide the MR ratio of only 5% at 4.2K.